1. The Field of the Invention
This invention relates generally to lightweight concrete block and a method and apparatus for making lightweight concrete block, and more particularly to a method and apparatus for making lightweight concrete blocks that are capable of being dry stacked and have an impact resistant outer surface.
2. Background
The use of lightweight concrete for building construction has been known for decades. Aerated, lightweight concrete has many desirable properties for use in the building construction industry. For example, it is typically easier to handle because of its decreased weight compared to conventional concrete structures. Furthermore, aerated lightweight concrete often has an xe2x80x9cR valuexe2x80x9d or insulative properties that eliminate or substantially decrease the need for additional insulation. Aerated lightweight concrete is also fire resistant so that any buildings built with such materials are less likely to be destroyed by fire.
Aerated, lightweight concrete is typically formed by one of two methods. One method involves mixing cement with an aerated foaming agent to form a cementitious slurry having air cells entrained therein. For example, as disclosed in U.S. Pat. No. 3,062,669 to Dilnot, a lightweight concrete is formed by combining Portland cement, ground silica, fibers, sodium silicate, water and a stable, preformed foam prepared by incorporating air into a hydrolyzed protein foaming agent. Similarly, in U.S. Pat. No. 3,867,159 to Ergene, cellular concrete structures are made by mixing water, cement, and a foam into a foamed cementitious slurry which is then cast in a mold and cured.
Another method includes adding alumina powder to the cement mixture. The alumina powder reacts with the cement mixture to form gas bubbles which cause aeration of the cement mixture. In order to form individual blocks of the lightweight concrete, the aerated cement mixture is often poured into a mold and allowed to harden around the air cells to form an aerated, lightweight concrete block.
Several approaches in the art have been employed to form aerated, lightweight concrete building units that are suitable for building purposes. In order for the lightweight concrete building units to be suitable for building purposes, they must have sufficient structural integrity, e.g., compressive strength, to meet building code requirements and they should be uniform in size and shape to be practical for use in the construction industry. In addition, the building units must be manufacturable in an efficient enough manner and in sufficient quantities to support demands required by the building construction industry. As such, one method known in the art of producing individual blocks is to form larger blocks of lightweight concrete and then cut the larger blocks into smaller building units while the cement is still in a partially cured or xe2x80x9cgreenxe2x80x9d state.
When employing cutting methods to form smaller building units, whether the initial larger block is formed by using alumina powder to cause the cement mixture to xe2x80x9crisexe2x80x9d or the initial block is formed by forming a foamed cement slurry by adding a stable foam to the mixture, the resultant cement slurry is poured into a large mold and allowed to at least partially cure into a relatively large block. For cement slurries that xe2x80x9crise,xe2x80x9d the height of the block within the mold is dependent upon the amount of aeration or gas generation that occurs within the cement mixture and the amount of gas that is entrained within the cement mixture when the mixture begins to harden. For cement slurries to which a stable foam has been added prior to being poured into a mold, the height of the block in the mold is determined by the amount of prefoamed cement slurry poured into the mold and the amount of air that escapes from the cement slurry before the viscosity of the cement slurry increases to a point where the air cells can no longer migrate within the mixture. Once the cement has hardened or cured to a degree where the formed block can be handled, the block may be removed from the mold and cut into smaller blocks of a desired size and shape. Because the height of the initially formed block is somewhat unpredictable, there is often significant amounts of scrap material produced during such block forming processes. That is, it is often the case that at least a top layer of the initially formed block is wasted. Examples of cutting apparatuses for cutting larger blocks into smaller building units are described in U.S. Pat. No. 4,174,936 to Goransson and U.S. Pat. No. 4,528,883 to Goransson et al.
When casting aerated cement compositions, it is common to find that the density of the block formed varies from top to bottom. That is, prior to solidification of the cement slurry, the gas cells migrate to the top of the block resulting in a block that has a greater density nearer the bottom of the block. Accordingly, individual building units that are cut from a larger block will vary in density resulting in blocks of varying structural strength and weight. In order to compensate for the varying density in individual building units, the density of the entire composition of the aerated slurry must be increased so that the least dense building nits have sufficient structural integrity.
In processes where the individual building units are cut from a larger block, a mortar or some other binding agent must be employed in order to use the building units for construction purposes. In addition, because each building unit has an outer surface that is comprised of open cells, water is easily absorbed into the building units. As such, the surface of the individual building units must typically be treated with a water repellant material to prevent water from absorbing into the block. This is especially important in colder climates where absorption of water can cause the building units to fracture as the water therein expands during solidification. Because of this known phenomenon, such building materials are often required to pass a xe2x80x9cfreeze/thaw test.xe2x80x9d In the freeze/thaw test, the material is submersed in water for an extended period of time and then frozen. If the material cracks, crumbles, or is otherwise structurally compromised, the material will not be approved for use in construction.
other methods for forming individual building units include individual casting in a mold. For example, as shown in U.S. Pat. No. 5,522,685 to John, the building units are each formed by pouring lightweight cement slurry into a mold to form a cast body and then combining pairs of cast bodies into individual building units. In U.S. Pat. No. 4,372,092 to Lopez, modular panels are individually formed by pouring a cement slurry into a single panel mold having desired components incorporated therein. Such methods for forming individual building units are typically not very efficient at producing large quantities of building units in a relatively short period of time. As such, the cost per unit is relatively high compared to conventional construction materials resulting in products that have not been very commercially successful.
One approach in the art to overcome the foregoing disadvantages with prior art systems is disclosed in U.S. Pat. Nos. 5,457,926 and 5,775,047 to Jensen, the inventor of the present invention, each of which are herein incorporated by this reference. In both references, a lightweight interlocking building block is disclosed in which the blocks may be stacked without the use of mortar. U.S. Pat. No. 5,775,047 teaches that the size of the bubbles entrained in the slurry produce a block having desired compressive and shear strengths. Neither reference, however, describes the method or apparatuses for forming such blocks. Furthermore, there is no teaching of the method or apparatus by which such blocks can be formed in a uniform manner to produce building units having substantially equal dimensions and relatively equivalent densities.
As such it would be advantageous to provide a method and apparatus for forming individual building units in an efficient and cost effective manner.
It would also be advantageous to provide a method and apparatus for forming individual building units having substantially uniform dimensions and relatively consistent densities for each building unit produced.
It would be a further advantage to provide a method and apparatus for forming individual building units in which the building units can be dry stacked without the need for mortar.
It would still be a further advantage to provide a method and apparatus for forming individual building units in which the building units have an impact resistant outer surface.
It would be yet another advantage to provide a method and apparatus for forming individual building units in which the density of the building units can be altered while maintaining the dimensions of the building units.
It would be another advantage to provide a method and apparatus for forming individual building units in which the density of the building units is relatively low while maintaining structural integrity sufficient to meet or exceed building requirements.
It would also be advantageous to provide a method and apparatus for forming individual building units in which the system for forming such building units is fully automated.
Accordingly, a method and apparatus for forming individual building units is provided, generally comprising an ingredient measuring apparatus, an automated cement slurry mixing apparatus, an automated cement slurry delivery apparatus, and at least one mold. The slurry is preferably comprised of cement, sand, water and foam. In a preferred embodiment, fibers are also added to the slurry to increase the structural integrity of the building units. Various aspects of the invention include the ability to control the amount of cementitious slurry in each batch in order to control to the extent possible the quantity of cementitious slurry poured into each mold and thus the density of the resulting building units and the ability to precisely control the dimensions of each building unit produced.
In a preferred embodiment of the present invention, an automated process for forming lightweight concrete building units is provided. The method includes providing a plurality of molds that are preferably laid out in rows and positioned on either side of a weighing and mixing device. Similarly, the molds may be placed on a turntable and sequentially positioned beneath the mixing device to receive batches of cementitious slurry. Each mold is preferably capable of simultaneously forming a plurality of building units.
Preferably, batches of cementitious slurry are individually mixed in order to control the properties, such as the density, of each building unit formed. To create a first single batch of cementitious slurry, the ingredients are individually measured and combined into a mixing device. The ingredients are then mixed for a period of time until a cementitious slurry is adequately formed. While the ingredients are being mixed, the resulting cementitious slurry is being conveyed or transported to one of the molds. In addition, while the first batch is mixing, a second batch of ingredients is being weighed in order to be dispensed into the mixing device after the first batch is dispensed from the mixing device.
Upon mixing of the second batch, the second batch is transported and poured into another mold while a third batch of ingredients is being measured for dispensing into the mixing device and for mixing to form a third batch of cementitious slurry. Additional batches are mixed, transported, and poured into additional molds until the first batch has cured to at least a point where the building units within the mold can be handled.
In another preferred embodiment, while the first batch is being transported and poured into one mold, a second batch of ingredients is being dispensed into the mixing device and mixed to form a second batch of a cementitious slurry.
After the building units have cured to a state where the building units are rigid enough to be handled, the building units are removed from their respective molds and transported to a storage area. This process is preferably continuously repeated so that as building units are removed from their respective mold, the mold is prepared, as by rinsing and/or applying a release agent (e.g., oil), for receiving another batch of cementitious slurry. As such the present invention provides a process where there is relatively little time when a particular mold in the process does not contain a batch of cement, except of course when the process is first started and all of the molds are initially empty.
In a preferred embodiment, the number of molds is dependent upon the process time. Preferably, however, the number of molds is approximately equal to the time between the start of a mix cycle for a batch of cementitious slurry and when the building units resulting from the batch are removed from their respective mold, divided by the time it takes to mix each batch and pour each batch into an individual mold. Such a process is highly advantageous as it maximizes the usage of each mold and maximizes the efficiency of the process.
In another preferred embodiment, the ingredients are measured by weight in which a first dry ingredient is poured onto a scale and weighed to approximately a first weight. A second dry ingredient is then added to the scale and weighed to a second weight. Additional ingredients as desired may be weighed and added to the mixture in a similar manner. The dry ingredients are then poured into a mixer to which metered amounts of foam and water are added in sufficient quantities to form a cementitious slurry having desired characteristics. Preferably, the amount of water and foam are metered by injecting these liquid ingredients for predetermined timed periods. By knowing the rate of flow (i.e., flow volume per unit time) for such liquid ingredients, relatively precise amounts can be metered into the mixture. In a preferred embodiment, the water is added to the mixing device prior to the addition of any dry ingredients in order to rinse the inside of the mixing device between batches.
One of the difficulties known in the art has been the inability to form batches of cementitious slurry with relatively consistent characteristics (i.e., density, even dispersion of aeration, quantities of ingredients, etc.) The present invention, however, solves these problems by using weighted amounts of dry ingredients and metered amounts of liquid ingredients to obtain relatively consistent quantities of ingredients in each batch. For controlling the quantities of the dry ingredients, which quantities typically have a greater affect on the characteristics of the finished product, each ingredient is metered at a first rate to approximately 90 percent of the desired weight (e.g., 90 percent). Each ingredient is then metered at a second slower rate to the desired weight. As such, the quantity of each ingredient can be more precisely controlled by adding the last amount of each ingredient at a slower rate without significantly decreasing the overall time it takes for each ingredient to be properly measured.
In a preferred embodiment, in order to further reduce the number of molds required while maximizing usage of the mixing device, the temperature of the cementitious slurry is kept at an elevated temperature of preferably about 80-120 degrees Fahrenheit. This temperature range is dependent upon the characteristics of the foaming agent selected. Specifically, foaming agents typically have a critical temperature above which the foaming agent will fail to produce a foam. In addition, there is typically a critical range of temperatures below the critical temperature at which the foaming agent will foam to some extent. For example, at a lower end of this critical temperature range, the foaming agent will foam quite well, while at a temperature nearer but less than the critical temperature the ability of the foaming agent to foam is reduced. Preferably, the foamed cementitious slurry is mixed at a temperature at or near the lower end of the critical temperature range to maximize foaming efficiency.
There is also often a second critical temperature of the foaming agent in slurry above which the foam will quickly destabilize (i.e., collapse). Often, but not necessarily, this second critical temperature is at or near the first critical temperature. Because, however, the foam will be mixed with the cementitious slurry, the slurry itself affects the temperature at which the foam will destabilize. In order to locally destabilize the foam at the interface between the mold and the slurry to create an outer hardened shell around each building unit produced, the mold is preferably heated above this second critical temperature or at least above the first critical temperature to obtain a desired shell thickness in the block. Knowing the temperature sensitive characteristics of the foaming agent (both alone and when mixed with a cement slurry) allows the other process parameters to be set to maximize the foaming efficiency of the foaming agent while encouraging quicker curing of the cement slurry and formation of an outer shell by the addition of heat. Those skilled in the art, will appreciate that the temperature may be kept at other temperatures depending upon the temperature sensitivity of the characteristics of the ingredients and the desired cure time.
Preferably, the temperature of the process is increased or decreased depending on the characteristics of the foaming agent used and the temperature at which the foam will maintain its air cells without significant breakdown. Heating of the cementitious slurry is desirable to decrease the cure time for each batch since the rate of cure for conventional cement products is typically proportional to the temperature. An objective of the present invention is to maximize the cure rate by maintaining the slurry at a temperature that is as near as possible to the critical temperature of the foam without compromising the stability of the foam. However, those skilled in the art will appreciate that lower temperatures may be employed in accordance with the present invention even though such lower slurry temperatures result in slower cure rates and may allow additional migration and coalescing of air cells during the curing process. Likewise, higher temperatures may be employed if a proper foaming agent is selected that can withstand higher temperatures.
In a preferred embodiment, a foaming agent is selected that has a critical temperature of approximately 110 to 120 degrees Fahrenheit (i.e., the temperature at which the resulting foam breaks down and its air cells collapse). The water that is added to the mixture is preferably heated to approximately 100 degrees Fahrenheit so that the stability of the foam is not compromised. When mixed with the dry ingredients (e.g., cement and sand), the mixture equalizes to approximately 80-90 degrees Fahrenheit. Finally, the mold is heated to approximately 170-220 degrees Fahrenheit. This temperature is preferred as it sufficiently collapses the air cells without substantially affecting the integrity of the cement. However, the higher the temperature of the mold, the thicker the dense outer layer of the building unit becomes. As such, the foaming agent remains stable during the mixing process as the heated slurry is below the critical temperature of the foam. As the cementitious slurry is poured into the mold, the air cells in the slurry that contact the surface of the heated mold collapse producing an outer layer of cement with fewer air cells. The use of a heated cementitious slurry in conjunction with the aid of the heated mold quickly cures the slurry such that the entrained air cells are prevented from coalescing, interconnecting, or migrating. Thus, the air cells remain evenly dispersed throughout the slurry. The hardened outer shell produced by the heated mold produces a dense outer layer that reduces the ability of the cement to draw in water by capillary action.
Preferably, in accordance with the method of the present invention, the batch of cementitious slurry is mixed as it is transported to a mold, such that the mixing time does not significantly delay the process. As such, the mixing device itself is transported to a mold while mixing is in progress and the slurry is poured into the mold upon arrival at the mold. Mixing the cementitious slurry while it is being transported to the mold keeps the cementitious slurry workable to prevent the slurry form setting during transport. After dispensing the slurry into a mold, the mixing device returns to the ingredient metering apparatus to receive a new batch of ingredients for mixing. The present invention thus utilizes mixing time during transport and allows each batch to be mixed for substantially the same amount of time for each mold. Maintaining consistent mixing times while keeping the transport time relatively short, allows each batch to cure an equal amount in each mold.
In a preferred embodiment, the first batch is transported at a first speed to within a relatively short distance of the first mold and then transported at a second slower speed to a substantially precise position relative to the first mold. As such, the position of the mixing device can be relatively precisely controlled while increasing the speed of the overall transporting operation. This transporting operation is repeated for each of the other molds.
Preferably, the cementitious slurry is comprised of sand, cement and water. In yet another preferred embodiment, fibers are added to the slurry to add structural integrity to the finished building units. In still another preferred embodiment, an additional cement is added to the mixture wherein the additional cement is a quick setting cement that decreases the curing time of each building unit. Utilizing such a dual cement mixture creates a cementitious composition that has the structural properties and cost savings of longer setting cement while taking advantage of the decrease in curing time resulting from the addition of the quicker setting cement. Decreasing the curing time of the cementitious slurry decreases the number of molds required for the process in accordance with the present invention in which molds are systematically and continuously filled while slurry batches in other molds are curing.
After the individual building units have cured to a point where they can be safely handled, if necessary, the building units are transported to a hydration station where water is applied to the building units in order to properly hydrate the cement. Of course, the cement may already contain enough water for proper hydration without requiring additional water. In addition, it is also contemplated in accordance with the present invention that the building units may be placed in an autoclave to aid the curing process.
In yet another preferred embodiment of the present invention, the aerated cementitious slurry is compressed while inside the mold. More specifically, the slurry is compressed a relatively precise amount to form an individual building unit having substantially precise dimensions. As such, each building unit produced will have precisely the same dimensions. Indeed, building units manufactured in accordance with the present invention are capable of having a tolerance of +/xe2x88x920.03 inches. Maintaining control over the dimensions of the finished building units allows the building units to be dry stacked without resulting in appreciable variations in wall height as the building units are stacked. That is, using building units of substantially equal dimensions to build a structure results in walls that do not substantially vary in height along their length.
Controlling the dimension of each building unit produced is preferably accomplished by compressing the aerated cementitious slurry within the mold in at least one direction such that a precise internal volume within the mold is achieved to form lightweight concrete building units having substantially precise volumes and dimensions when cured. In addition, by metering the ingredients in such a relatively precise manner as herein disclosed, the density of each block will be substantially uniform for each block.
Compressing the cementitious slurry also causes air cells proximate outer surfaces of the lightweight building unit to collapse resulting in a lightweight building unit having an impact resistant outer surface or shell when cured. The use of compression techniques in accordance with the principles of the present invention may also be employed to imprint a texture or pattern on at least one outer surface of the lightweight building unit.
In this process it is also beneficial to cause the cementitious slurry to cure to a handleable point as quickly as possible in order to maintain the aeration that is entrained within the cementitious slurry in the cured building unit. One way of decreasing the cure time of the cementitious slurry is to heat the water that is added to the mixture (e.g., to a temperature at or near 140 degrees Fahrenheit). In addition, heating the molds (e.g., to a temperature at or above approximately 180 degrees Fahrenheit) also speeds the cure time of each building unit. In fact, building units manufactured in accordance with the principles of the present invention can cure to a handleable state in approximately ten (10) minutes or less. It is preferable that the molds be heated by circulating heated water through the molds. In a preferred embodiment, the heated water that is added to the cementitious slurry and the water utilized to heat the molds is supplied from a common source.
In still another preferred embodiment, the foaming agent utilized to aerate the cementitious slurry has a first critical temperature of approximately 100 degrees Fahrenheit (i.e., the temperature above which the foaming agent will not produce a stable foam). The foaming agent is heated to about 80 degrees Fahrenheit, agitated into a foam, and added to the cementitious slurry for mixing therein. In a preferred embodiment, the heated water added to the foaming agent is from the same source of heated water that supplies water to the mix for forming each batch of cementitious slurry so long as the temperature of the water is not above the fist critical temperature of the foaming agent. The mold is preferably heated above a second critical temperature of the foaming agent (i.e., the critical temperature of the foam itself or the temperature at which the foam destabilizes).
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.